1. Field of the Invention
The present invention relates to an angle computation method for a variable reluctance resolver (hereinafter VR resolver) which enables an accurate angular detection even when unignorable errors arise from various portions of the resolver, and to an angle computation apparatus for performing the angle computation method.
2. Description of the Related Art
There exist a variety of resolvers, which differ in terms of operation scheme, coil structure, wiring, etc. For example, a VR resolver of a one phase excitation/two phase output includes a stator on which are wound an excitation coil, a SIN voltage output coil, and a COS voltage output coil; and a rotor which has a plurality of salient poles and which, upon rotation within the stator, changes the gap permeance between the two output coils in accordance with a sine function, which is a function of a rotational angle θ. The SIN voltage output coil outputs a sine output voltage, and the COS voltage output coil, which is formed with a phase difference of 90 degrees (electrical angle) with respect to the SIN voltage output coil, outputs a cosine output voltage. The sine and cosine output voltages are induced because of variation in magnetic flux upon rotation of the rotor, and the rotational position of the rotor is detected on the basis of these induced voltages.
The stator of such a VR resolver has a large number of magnetic pole teeth. Each output voltage of the resolver is obtained as the sum total of output voltages of coils wound around corresponding magnetic pole teeth. The voltage output from the SIN voltage output coil for extracting a sine output voltage at each magnetic pole tooth is represented by α+β sin θ. The value of sin θ is determined mainly from the shape of the rotor, whereas the values of α and β are determined mainly from magnetic paths and coils of the stator. When the outputs of the respective magnetic pole teeth are represented by α1+β1 sin θ1, −α2+β2 sin θ2, . . . αm+βm sin θm, the output of the resolver is represented as follows.
            ∑              n        =        1            m        ⁢          (                        α          n                +                              β            n                    ⁢                                          ⁢          sin          ⁢                                          ⁢                      θ            n                              )        =      K    ⁢                  ⁢    sin    ⁢                  ⁢    ϕ  
The voltage output from the COS voltage output coil for extracting a cosine output voltage can be represented in an analogous manner.
An angle detector used in a resolver and using computation means such as a microcomputer computes an angle, while using the received voltages of the resolver as they are.
In an ideal case where the respective magnetic pole teeth have equal output amplitudes, the value of α is zero, so that the output voltage (Vsin) of the SIN voltage output coil and the output voltage (Vcos) of the COS voltage output coil can be represented as follows:Vsin=B sin θ, andVcos=D cos θ.
However, because of transformation ratio of resolver outputs, short circuits of output coils, run-out of the rotor, variation in magnetic characteristics among stators, variation in coils among stators, and other factors, output error increases, and an accuracy of angle detection deteriorates.
Although such a VR resolver does not generate signals which have clear relations of sin θ and cos θ, respectively, with respect to the angular position θ of the shaft, it generates signals which have a high degree of reproducibility and which vary as a function of the shaft position. Angle correction has been performed through utilization of this feature. Specifically, a reference table is prepared, and sin θ and cos θ values are corrected with reference to the reference table. The VR resolver is driven at a constant speed, and distorted sine and cosine signals are recorded. These distorted signals are passed through a Fourier transformerin order to extract basic sine and cosine waves. Subsequently, these basic waves are compared with the original, distorted sine and cosine signals output from the resolver, and are used to create a correction reference table for obtaining correction values (see, for example, Japanese Patent Application Laid-Open (kokai) No. 11-51692).
The method of obtaining proper correction values with reference to such a correction reference table has a drawback in that the table must be re-created in accordance with the details of an error, and the procedure of creating the table is complicated and time consuming.
When output errors stemming from the above-described various causes are taken into consideration, the output voltage (Vsin) of the SIN voltage output coil and the output voltage (Vcos) of the COS voltage output coil can be represented as follows.Vsin=A+B sin θ  Eq. 1Vcos=C+D cos θ  Eq. 2That is, the output voltage (Vsin) of the SIN voltage output coil and the output voltage (Vcos) of the COS voltage output coil are offset by DC components A and C, respectively. When, as in the conventional technique, an angle is computed from such output voltages, computation errors are produced under the influence of the DC components A and C.
In order to improve accuracy, the number of turns of coils can be adjusted such that the values of the terms “A” and “C” of Eqs. 1 and 2, which appear as output errors, become zero. However, since the number of turns cannot be increased infinitely, in some cases, the adjustment cannot reduce to zero the values of the terms “A” and “C.” Moreover, the number of turns does not necessarily become the same among individual resolvers, because of variation thereamong.